576 THE ICE AGE IN NORTH AMERICA. 



glacial " forest-beds.* These forest-beds and vegetal de- 

 posits occur over a wide area, and in places have glacial de- 

 posits both under them and over them. The first supposi- 

 tion with regard to them was that these various forest-beds 

 were contemporaneous, and indicated a general retreat of 

 the ice after its first invasion of North America until it had 

 entirely disappeared or lingered only in the Canadian high- 

 lands ; whereupon there was a readvance of the ice, over- 

 whelming the forests and other vegetal deposits which had 

 collected in kettle-holes and other depressions, and burying 

 them beneath a second sheet of ground-moraine, where they 

 are opened to present inspection whenever wells penetrate 

 them or eroding streams expose them on their banks. But 

 it is not clear that these interglacial forest-beds might not 

 originate in front of the margin of the slowly retreating ice 

 if only there were comparatively brief periods of readvance 

 along successive lines of latitude. Thus they may belong to 

 various times of oscillation, both during the general advance 

 and during the general retreat of the glacier. If, for ex- 

 ample, at any time during the period of advance there had 

 been a retrocession of the ice-front for a short distance, for- 

 ests and vegetable growth would soon have spread over the 

 marginal belt from which the ice had retreated, and, upon 

 a readvance, these would be overwhelmed and covered with 

 a new stratum of glacial deposition. In case of some of 

 the peat beds, it is probably necessary to suppose that they 

 were formed where they are, and are really interglacial ; but, 

 in case of many of the fragments and logs of wood found 

 in the glacial deposit, we are not compelled to suppose an 

 interglacial origin. Wood will stand transportation in the 

 ground-moraine almost as well as bowlders, and it is by no 

 means certain that much of the timber found in the till may 



* See Chamberlin, " Geology of Wisconsin," vol. i, chap, xv, especially 

 pp. 271-291 ; "Driftless Area," pp. 211-216; N. H. Winchell in " Proceedings 

 of the American Association for the Advancement of Science," vol. xxiv, 1875, 

 pp. B, 43-56 ; " Geology of Minnesota,"' vol. i of the " Final Report," pp. 363 

 et seq. ; J. S. Newberry, "Geological Survey of Ohio," vol. ii, pp. 30-33. 



